Error correction of amplifiers grows ever more important as frequency allocation, frequency reuse, and RF interference (RFI) become more prevalent. In the amplification of electronic message signals one paramount consideration is to maintain signal fidelity, while another paramount consideration is to minimize the operating power requirements. Those two factors are in conflict with each other. High efficiency amplifiers will typically have high signal distortion, but high-fidelity amplifiers typically require high operating power.
All amplifiers produce distortion products, or error signals, as a part of the amplification process. The amount of error varies, but is primarily due to the operational class of the amplifier. Class C amplifiers, while very efficient, generate significant error signals. Class AB amplifiers operate somewhat less efficiently, producing less error than a Class C amplifier operating at similar power levels. Class A amplifiers provide the lowest level of error signal, but at a higher cost such that the power efficiency of the amplifier is very poor. As a tradeoff, one can use a Class AB amplifier for a specific application such as wireless telephony transmission, and utilize associated circuitry operating in the feed forward cancellation mode for reducing the error components generated by the amplifier. This provides reduced error levels at a reasonable level of operational efficiency.
The feed forward error control concept was originated in the 1920's by Harold S. Black and described in his U.S. Pat. No. 1,686,792 issued Oct. 9, 1929. The concept is more fully described in an article entitled "A Microwave Feed-Forward Experiment", by H. Seidel, published in The Bell System Technical Journal, Vol. 50, No. 9, Nov., 1971. Its important properties are that it incorporates time, phase and amplitude compensation to reduce error signals produced by the amplifier. Compensation of these three parameters allows operation at much higher frequencies, and over much greater bandwidths, than other types of error control such as negative feedback. Also, because time compensation is incorporated into the system, the ultimate performance of the system becomes dependent upon the physical component variations, and not upon limitations due to transit time and associated phase shift through the system.
The feed forward amplifier system utilizes a main amplifier having a high power efficiency, and which is permitted to operate with high signal distortion. Associated circuits then observe and measure the distortion or error, and produce a correction signal which is added into the final output of the amplifier system so as to offset or counteract the signal distortion. A comparison is made between input and output signals of the main amplifier in order to provide an error signal, and a separate error amplifier is utilized to amplify the error signal before its re-insertion with appropriate polarity into the main amplifier path. Feed forward amplifier systems typically have poor power efficiency, however.
The feed forward amplification process involves signal amplification, recognition and amplification of the errors or undesired signals, and combination of properly compensated error signals with the distorted output signal of the main amplifier so as to produce a corrected final output signal in which the level of the error signals is reduced by cancellation or destructive interference. Associated circuitry includes an error correction circuit for detecting message signal error in the operation of the main amplifier and producing an amplified message signal error which is then subtracted from the amplified and distorted message signal output of the main amplifier to produce a corrected final output signal.
Signal amplification in the main amplifier not only produces signal distortion or error, but also delays the signal as it flows along the main signal path. A comparison loop that compares the input and the output of the main amplifier must have an artificial time delay inserted into it to balance the inherent time delay that could not be avoided in the main signal path.
As is well known, the performance of amplifiers and other comparable communication circuits will drift as a function of power supply voltage, temperature changes, time, and other environmental factors. The typical reason for such a drift in performance is that the actual or effective values of some of the circuit components change. It is therefore a well known expedient to provide such circuitry with adaptive controls that respond to changing circuit conditions so as to maintain circuit performance at as near a constant level as possible. Most commonly, these adaptive controls involve negative feedback circuitry, often with a test tone for detecting and drift in the circuit performance.
Thus a typical feed forward amplifier system for amplifying radio frequency telephony signals includes a main amplifier, an error correction circuit including an error amplifier for producing an amplified message signal error that is then subtracted from the amplified and distorted message signal output of the main amplifier for cancelling error prior to a final output circuit, and a negative feedback circuit coupled to the final output circuit for compensating and minimizing variation or drift in the operation of the error amplifier.
In modern communication circuits there is a need for efficient, high fidelity amplification of information signals occupying wide frequency bands. The information may require processing in analog form, in digital form, or in some combination or variation thereof. At the same time, it is important to maintain high fidelity throughout the operating frequency band.
In modern cellular telephone systems the base station or repeater amplifiers may be required to amplify signals occupying a bandwidth that is up to three percent of an underlying carrier frequency. Personal Communication System (PCS) equipment may require effective operation over bandwidths of more than three per cent of an underlying carrier frequency. Consistent, reliable, high fidelity amplification throughout such a frequency band is inherently difficult to achieve.
Many engineering techniques besides amplification and the balancing of time delays are required to implement an effective feed forward amplifier system. Signal amplification and signal sampling circuits may be adjusted to provide balanced signals where needed. Signal splitting may be utilized to provide a sample of a particular signal without destroying the original. Coupling circuits may be utilized to combine signals either additively or substractively, as may be required. Phase correction or stabilization may be provided at appropriate points in the circuitry. Use of these techniques in proper combination then makes it possible to produce an amplified error signal that, when added to the amplified message signal, will provide an output signal from the feed forward system that is essentially free of signal distortion.
To achieve that result, however, it has generally been necessary to employ an error amplifier that operates at a power level roughly as high as the power level of the main amplifier. Even though the error signal being amplified in the error amplifier is of smaller magnitude than the signal in the main signal path, high operating power is required in order for the error amplifier to maintain a Class A operation of satisfactory linearity. Thus, the total operating power consumed may be approximately double the power consumed by the main amplifier alone.